Abstract

Two basement thrusts form the productive A and B structures at Gooseberry Field on the west flank of the Bighorn Basin (T. 47 N. R. 100 W.). Analysis of new seismic data indicate that A and B are the result of basement backthrusts in the hanging-wall of the west-dipping Oregon Basin Thrust. The A basement fault dips to the east at 55° and has 1,000 ft of structural relief. It is directly overlain by west-dipping thrusts in the sedimentary cover which add an additional 800 ft of structural relief to A. The shallower thrusts form a detachment system consisting of faults that cut up-section from the base of the Bighorn Dolomite through the Thermopolis Formation. Fault-propagation folding accompanied detachment faulting. Although early motion along the A basement fault localized the thrust ramps of the overlying detachments, its motion continued after the shallower thrusting ended. In map view, A is truncated in the south by a north-dipping thrust that aligns with the Tensleep Lineament. B is a fault-cored, asymmetric anticline/syncline pair with a maximum of 3,400 ft of structural relief. Geometric modeling, using a new technique that balances layer-parallel shear in the sedimentary cover with simple shear in basement, indicates that the B basement fault dips to the east at 60°, is listric, and flattens at a depth of approximately 65,000 ft where it intersects the Oregon Basin Thrust.